1. Field of the Invention
The present invention relates to automatic control of blood oxygen saturation (S.sub.a O.sub.2) in a patient by adjustment of the fractional amount of oxygen inspired (FIO.sub.2) by the patient, and more particularly, to a feedback control loop for a mechanical ventilator including a non-invasive S.sub.a O.sub.2 monitor in the feedback path for developing an adaptive control signal which controls the inspired gas blender of the ventilator.
2. Description of the Prior Art
Devices for controlling the oxygen content of the blood by controlling the fraction of oxygen breathed by a patient are well known. For example, U.S. Pat. No. 2,414,747 issued to Harry M. Kirschbaum on Jan. 21, 1947 shows a method and apparatus for controlling the oxygen content of the blood of living animals which discloses control of blood oxygen content by the use of an ear oximeter which produces a signal used to control the proportion of inspired oxygen. As the oximeters direct a beam of light through a capillary bed in the ear, the characteristics of the light become modified by the color of the blood that intercepts its path. Thus, the change in oxygen levels of the blood are detected non-invasively and electrical signals are generated, amplified and used to control the oxygen supply delivered to a patient.
Numerous improvements have been made since that time wherein better matching of oxygen delivery to the needs of the patient have been made such as shown in U.S. Pat. No. 3,734,091 to Ronald H. Taplin issued on May 22, 1973. Taplin discloses an optical oximeter and a temporary oxygen-deficient mixture (anoxic) to control blood oxygen saturation. Thus, to prevent excessive oxygen levels, Taplin discloses limiting the inspired oxygen by intermittently providing the anoxic mixture each time the oxygen saturation of the blood reaches a predetermined percentage level.
U.S. Pat. No. 4,889,116 issued to Taube on Dec. 26, 1989 discloses one type of adaptive controller for adjusting the fraction of oxygen inspired by a patient. The controller utilizes a pulse oximeter connected by an optical sensor to the patient for measuring the patient's blood hemoglobin saturation (S.sub.p O.sub.2) and pulse rate. These signals from the oximeter are used by a calculator for determining the fractional amount of oxygen to be inspired by the patient. The calculated percentage of oxygen is provided to the patient so that the gas taken in by the patient automatically causes the blood in the patient to reach a predetermined hemoglobin saturation level in response to the patient's requirements as determined by changes in the S.sub.p O.sub.2 signal. However, the calculator is programmed to determine when there is an excess deviation of the patient's pulse rate, thereby indicating patient movement and the probability that the pulse oximeter will provide false S.sub.p O.sub.2 values during such patient movement. When an excess deviation in pulse rate is detected, the fractional amount of inspired oxygen is no longer responsive to the measured S.sub.p O.sub.2 value, but instead held constant until the excess deviation of the pulse rate has been terminated. Furthermore, a low S.sub.p O.sub.2 value, indicative of a depressed respiration (apnea) is also detected, and used to cause a preset higher percentage of inspired oxygen to be supplied to the patient until the depressed respiration of the patient has been terminated. Thus, responsive FIO.sub.2 adjustment is suspended during patient movement and apnea, and during this time fixed FIO.sub.2 levels are set.
In 1980, H. Katsuya and Y. Sakanashi published an article in the Journal of Clinical Monitoring 1989; 5:82-86 describing a method for evaluating pulmonary gas exchange using a pulse oximeter. They developed the concept of FI.sub.9x (where x is a single digit number) which is the fraction of inspired oxygen necessary to achieve a measured blood oxygen saturation equal to the value of 9x % (e.g. 98%). This experiment was carried out by periodically manually increasing or decreasing the FIO.sub.2 control of a gas blender portion of a ventilator until the S.sub.p O.sub.2 measurement reached the target percentage (e.g., 98%). The purpose of this experiment was to develop a diagnostic method to evaluate pulmonary gas exchange impairment. A high value of FI.sub.9x was associated with poor pulmonary gas exchange. In this publication, no mention was made of feedback of S.sub.p O.sub.2 values for automatic adjustment of FIO.sub.2. The present invention recognizes that the pulmonary impairment of a patient can often change during treatment, thereby requiring a change or adaptation of the responsiveness of the FIO.sub.2 control loop.
Since adult and neonatal patients in intensive care units suffering from respiratory distress are at risk for developing hypoxemia or oxygen toxicity, certain safety precautions should be taken to prevent O.sub.2 under/overdose. In an attempt to maintain organ normoxia, appropriate clinical care often mandates ventilation with high FIO.sub.2, sometimes for several days. Long exposure to high concentrations of oxygen has been implicated in complications including exacerbation of respiratory distress and various central nervous system symptoms. In neonatal patients, oxygen toxicity may result in blindness from retrolentalfibroplasia. Thus, care should be taken to minimize the FIO.sub.2 exposure while maintaining adequate S.sub.p O.sub.2, so that the onset of these insidious complications can be delayed or avoided. Furthermore, artifacts (false output measurements) are commonly found in the pulse oximeter output due to patient movement and/or low blood perfusion in the area where the patient contacts the pulse oximeter sensor. Additionally, it is difficult to actually know what the arterial blood oxygen saturation percentage is from the S.sub.p O.sub.2 (pulse oximeter) measurement. Thus, careful construction of the S.sub.p O.sub.2 feedback control system is required.
It is an object of the present invention to provide a method and apparatus which minimizes the FIO.sub.2 of a patient while maintaining adequate S.sub.a O.sub.2 levels.
It is a further object of the invention to provide artifact rejection processing of the pulse oximeter which is tolerant of the expected false readings of S.sub.p O.sub.2, and furthermore, which is adaptive so as to gradually cause the FIO.sub.2 to increase as the frequency of the artifacts increases.
It is still a further object of the invention to provide an FIO.sub.2 feedback control loop which has a response (transfer characteristic) which is adaptive to the changing requirements of the patient.
It is an even further object of the invention to provide a safety sub-system for the FIO.sub.2 control system in order to prevent a failure in the feedback control system from causing injury to the patient due to extremely inappropriate levels of FIO.sub.2.